U.S. patent application number 14/110240 was filed with the patent office on 2014-04-03 for modified natural graphite particles.
This patent application is currently assigned to CHUO DENKI KOGYO CO., LTD.. The applicant listed for this patent is Tooru Fujiwara, Tatsuo Nagata, Noriyuki Negi, Katsuhiro Nishihara, Hiroshi Yamamoto. Invention is credited to Tooru Fujiwara, Tatsuo Nagata, Noriyuki Negi, Katsuhiro Nishihara, Hiroshi Yamamoto.
Application Number | 20140093781 14/110240 |
Document ID | / |
Family ID | 46969167 |
Filed Date | 2014-04-03 |
United States Patent
Application |
20140093781 |
Kind Code |
A1 |
Nishihara; Katsuhiro ; et
al. |
April 3, 2014 |
Modified Natural Graphite Particles
Abstract
A modified natural graphite material which provides a negative
plate having improved adhesion between a negative electrode mixture
and a current collector has a circularity of at least 0.92 and at
most 1.0 and an incident angle dependence S.sub.60/0 of the peak
intensity ratio of at least 0.5 and at most 0.7 as determined by
measurement of C K-edge X-ray absorption spectra using synchrotron
radiation as an excitation source. It preferably satisfies at least
one of the following conditions: an absolute specific gravity of at
least 2.25 (g/cm.sup.3), a tap density of at least 1.0 g/cm.sup.3
and at most 1.4 g/cm.sup.3, and linseed oil absorption of at least
20 cm.sup.3/100 g and at most 50 cm.sup.3/100 g. A carbonaceous
material may adhere to at least a portion of the surface of the
graphite particles.
Inventors: |
Nishihara; Katsuhiro;
(Osaka-shi, JP) ; Yamamoto; Hiroshi; (Osaka-shi,
JP) ; Nagata; Tatsuo; (Osaka-shi, JP) ; Negi;
Noriyuki; (Osaka-shi, JP) ; Fujiwara; Tooru;
(Myoko-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nishihara; Katsuhiro
Yamamoto; Hiroshi
Nagata; Tatsuo
Negi; Noriyuki
Fujiwara; Tooru |
Osaka-shi
Osaka-shi
Osaka-shi
Osaka-shi
Myoko-shi |
|
JP
JP
JP
JP
JP |
|
|
Assignee: |
CHUO DENKI KOGYO CO., LTD.
Myoko-shi
JP
|
Family ID: |
46969167 |
Appl. No.: |
14/110240 |
Filed: |
April 3, 2012 |
PCT Filed: |
April 3, 2012 |
PCT NO: |
PCT/JP2012/059059 |
371 Date: |
December 17, 2013 |
Current U.S.
Class: |
429/231.8 |
Current CPC
Class: |
C01P 2006/19 20130101;
C01B 32/21 20170801; H01M 4/587 20130101; H01M 4/133 20130101; H01M
4/366 20130101; C01P 2006/11 20130101; C01P 2006/10 20130101; H01M
10/052 20130101; Y02E 60/10 20130101 |
Class at
Publication: |
429/231.8 |
International
Class: |
H01M 4/587 20060101
H01M004/587 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 8, 2011 |
JP |
2011-086266 |
Claims
1. Modified natural graphite particles characterized by having a
circularity of at least 0.92 and having an incident angle
dependence S.sub.60/0 of the peak intensity ratio defined by the
following equation of at least 0.5 and at most 0.7, the peak
intensity ratio being determined by measurement of C K-edge X-ray
absorption spectra using synchrotron radiation as an excitation
source: S.sub.60/0=I.sub.60/I.sub.60 where
I.sub.60=B.sub.60/A.sub.60 I.sub.0=B.sub.0/A.sub.0 A.sub.60: the
intensity of the absorption peak assigned to the transition from
C-1s level to .pi.* level in a C K-edge X-ray absorption spectrum
of particles when measured with synchrotron radiation having an
angle of incidence of 60.degree.; B.sub.60: the intensity of the
absorption peak assigned to the transition from C-1s level to
.sigma.* level in a C K-edge X-ray absorption spectrum of particles
when measured with synchrotron radiation having an angle of
incidence of 60.degree.; A.sub.0: the intensity of the absorption
peak assigned to the transition from C-1s level to .pi.* level in a
C K-edge X-ray absorption spectrum of particles when measured with
synchrotron radiation having an angle of incidence of 0.degree.;
and B.sub.0: the intensity of the absorption peak assigned to the
transition from C-1s level to .sigma.* level in a C K-edge X-ray
absorption spectrum of particles when measured with synchrotron
radiation having an angle of incidence of 0.degree..
2. The modified natural graphite particles as claimed in claim 1
having an absolute specific gravity of at least 2.25
(g/cm.sup.3).
3. The modified natural graphite particles as claimed in claim 1
having a tap density of at least 1.0 g/cm.sup.3 and at most 1.4
g/cm.sup.3.
4. The modified natural graphite particles as claimed in claim 1
having linseed oil absorption of at least 20 cm.sup.3/100 g and at
most 50 cm.sup.3/100 g.
5. Carbon-deposited graphite particles comprising modified natural
graphite particles as claimed in claim 1 and a carbonaceous
material adhering to at least a portion of the surface of the
graphite particles.
Description
TECHNICAL FIELD
[0001] This invention relates to modified natural graphite
particles which are useful as a negative electrode active material
in a negative plate (negative electrode plate) of a non-aqueous
electrolyte secondary battery and particularly a lithium ion
secondary battery.
BACKGROUND ART
[0002] A negative plate for a non-aqueous electrolyte secondary
battery typified by a lithium ion secondary battery is prepared by
coating a current collector with a negative electrode mixture which
is formed by mixing at least a negative electrode active material
and a binder followed by compaction. The collector is often made of
a foil of copper or a copper alloy.
[0003] The negative electrode active material which is used is a
material capable of occluding cations (positive ions) such as
lithium ions therein at the time of charging. Typical materials
used for a negative electrode active material include graphite
substances, which have a layered crystal structure and thereby
allow cations to be occluded between the layers. Graphite
substances are broadly classified into natural graphite and
artificial graphite. In general, natural graphite is less expensive
than artificial graphite. Among types of natural graphite, those
which have a flake shape and hence a high aspect ratio have a high
degree of graphitization, which is indicative of crystallinity of
graphite, and are therefore expected to have a high
charge-discharge capacity (referred to below merely as capacity)
when used as a negative electrode active material. However, due to
the anisotropy of their shape, these types of natural graphite
having a high aspect ratio have shortcomings such as that they are
oriented when applied to a collector, they have a large initial
irreversible capacity, and they have a low packing density.
Accordingly, natural graphite particles having a high aspect ratio
are not used as an active material as is, but are normally used
after they are subjected to shape modification treatment.
[0004] Patent Document 1 and Non-Patent Document 1 disclose a
method for modifying the shape of graphite particles in which
Mechano Fusion (registered trademark) is used to modify the
particle shape into a disc shape. Patent Document 2 discloses a
method for spheroidizing graphite particles using a jet mill Patent
Documents 3 and 4 disclose a method for spheroidizing graphite
particles using a pin mill.
[0005] A binder, which is the other essential component of a
negative electrode mixture, serves to adhere particles of a
negative electrode active material to each other or to a collector.
As long as the required adhesion is obtained, it is desirable that
the binder have a high availability. In particular, in view of the
requirement in to recent years that the density of an electrode
(the capacity per unit volume of a negative electrode mixture) be
increased, there is a tendency to decrease the total amount of a
binder used in the mixture in order to minimize the amount of a
binder which exists between graphite particles without contributing
to adhesion of particles of a negative electrode active material to
each other or to a collector and which thereby interferes with
densification of an electrode.
PRIOR ART DOCUMENTS
Patent Documents
[0006] Patent Document 1: JP 2007-169160 A [0007] Patent Document
2: JP 11-263612 A [0008] Patent Document 3: JP 2003-238135 A [0009]
Patent Document 4: JP 2008-24588 A
Non-Patent Documents
[0009] [0010] Non-Patent Document 1: K. Ohzeki et al., Tanso
(Carbon), 2005, No. 217, pp. 99-103
SUMMARY OF THE INVENTION
[0011] When the amount of a binder used in a negative electrode
mixture which is used in the manufacture of a negative plate of a
non-aqueous electrolyte secondary battery is simply decreased, the
bonding strength of particles of a negative electrode active
material to each other or to a collector decreases, leading to the
problem that the negative electrode mixture drops off the collector
or negative plate during a manufacturing process of a negative
plate or during assembly of a battery. Particularly, when it is
desired to increase the speed of manufacture or assembly, due to a
tendency for the bending force or tensile force applied to the
layer of a negative electrode mixture formed on a collector or to a
negative plate to increase, the above problem that the negative
electrode mixture drops off tends to occur easily. When a negative
electrode mixture drops off, it produces not only a deterioration
of the product quality but also a decrease in product yield and a
significant decrease in productivity caused by stopping of a
production line. Considering that the use of non-aqueous
electrolyte secondary batteries for automobiles and electric power
storage as well as for consumer appliances is expanding, it is
important to reduce the manufacturing costs of the batteries by
improving productivity.
[0012] The object of the present invention is to solve the
above-mentioned problem by providing a natural graphite substance
capable of producing a negative plate having improved bonding
strength.
[0013] As a result of investigations aimed at achieving this
object, the present inventors obtained the following new
findings.
[0014] (A) Among the conventional treatment methods for modifying
the shape of natural graphite particles having a high aspect ratio,
a method in which a trituration-type grinding means such as Mechano
Fusion (registered trademark) is used cannot sufficiently
spheroidize graphite particles. Other shape modifying methods are
all focused on modification of the overall shape of graphite
particles. Therefore, although a reduction in anisotropy of the
shape can be achieved, the surfaces of the particles remain
rough.
[0015] (B) When graphite particles have a rough surface, if the
amount of a binder which is used is small, only protrusions on the
rough surface adhere to adjoining graphite particles or a current
collector. Accordingly, the bonding strength between graphite
particles or between a graphite particle and a collector decreases,
thereby causing a negative electrode mixture to easily drop off the
collector or a negative plate.
[0016] (C) In order to restrain a negative electrode mixture from
dropping off, it is efficient to improve (smoothen) the roughness
of the surface of graphite particles by a shape modifying treatment
of natural graphite.
[0017] (D) A preferable means for quantitatively assessing the
surface roughness of graphite particles is measurement of the
degree of orientation of graphite particles by means of C (=carbon)
K-edge XAS(=X-ray absorption spectroscopy) spectra combined with
measurement of the amount of absorption of linseed oil.
[0018] The present invention, which was completed based on the
above findings, is modified natural graphite particles
characterized by having a circularity of at least 0.92 and having
an incident angle dependence S.sub.60/0 of the peak intensity ratio
defined by the following equation of at least 0.5 and at most 0.7,
the peak intensity ratio being determined by measurement of C
K-edge (Carbon K-edge) X-ray absorption spectra using
SR(=synchrotron radiation) as an excitation source:
S.sub.60/0=I.sub.60/I.sub.0
where
[0019] I.sub.60=B.sub.60/A.sub.60
[0020] I.sub.0=B.sub.0/A.sub.0
[0021] A.sub.60: the intensity of the absorption peak assigned to
the transition from C-1s level to .pi.* level in a C K-edge X-ray
absorption spectrum of particles when measured with synchrotron
radiation having an angle of incidence of 60.degree.;
[0022] B.sub.60: the intensity of the absorption peak assigned to
the transition from C-1s level to .sigma.* level in a C K-edge
X-ray absorption spectrum of particles when measured with
synchrotron radiation having an angle of incidence of
60.degree.;
[0023] A.sub.0: the intensity of the absorption peak assigned to
the transition from C-1s level to .pi.* level in a C K-edge X-ray
absorption spectrum of particles when measured with synchrotron
radiation having an angle of incidence of 0.degree.; and
[0024] B.sub.0: the intensity of the absorption peak assigned to
the transition from C-1s level to .sigma.* level in a C K-edge
X-ray absorption spectrum of particles when measured with
synchrotron radiation having an angle of incidence of
0.degree..
[0025] Circularity is expressed by the following equation and has a
maximum value of 1:
Circularity=(Circumference of a circle having the same area as a
projected shape of a particle)/(Circumference of the projected
shape of the particle).
[0026] The projected shape is the shape obtained by projecting a
particle which is being measured on a two-dimensional plane. The
circumference of a circle having the same area as a projected shape
of a particle and the circumference of the projected shape of the
particle are determined by image processing of the image of the
projected shape.
[0027] In preferred embodiments, modified natural graphite
particles according to the present invention satisfy at least one
of the following conditions (a) to (c):
[0028] (a) they have an absolute specific gravity of at least 2.25
(g/cm.sup.3),
[0029] (b) they have a tap density of at least 1.0 g/cm.sup.3 and
at most 1.4 g/cm.sup.3, and
[0030] (c) they have linseed oil absorption of at least 20
cm.sup.3/100 g and at most 50 cm.sup.3/100 g.
[0031] The present invention also provides carbon-deposited
graphite particles comprising the above described modified natural
graphite particles and a carbonaceous material adhering to at least
a portion of the surface of the graphite particles.
[0032] Modified natural graphite particles according to the present
invention have a sufficiently smoothened surface, so they make it
possible to prepare a negative plate having an adequate bonding
strength between a negative electrode mixture and a current
collector even when the amount of a binder in the negative
electrode mixture is suppressed. Such a negative plate is of high
quality and can be manufactured with high productivity. As a
result, a non-aqueous electrolyte secondary battery having a
negative plate prepared using modified natural graphite particles
according to the present invention is also of high quality and can
be manufactured with high productivity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a diagram showing the principle of measurement by
X-ray absorption spectroscopy employed in the present invention as
compared to X-ray photoelectron spectroscopy (XPS).
[0034] FIG. 2 is a diagram showing the basic concept of a method
for measuring X-ray absorption spectroscopy by means of synchrotron
radiation used in the present invention.
[0035] FIG. 3 shows C K-edge NEXAFS spectra when carbon is
irradiated with synchrotron radiation having different angles of
incidence (0.degree., 30.degree., and 60.degree.), wherein FIG.
3(A) is a case in which the carbon is a single crystal of HOPG
(Highly Oriented Pyrolytic Graphite), and FIG. 3(B) is a case in
which the carbon is an amorphous carbon vapor deposition film (with
a thickness of 10 nm).
[0036] FIG. 4 is a diagram explaining a method for quantitatively
assessing the degree of orientation of surface graphite crystals
according to the present invention by taking a case in which the
sample is HOPG as an example.
[0037] FIG. 5(A) is an explanatory diagram conceptually showing the
process in which natural graphite is granulated by spheroidizing
treatment, FIG. 5(B) shows to a SEM observation image of
spheroidized graphite in the course of spheroidizing treatment, and
FIG. 5(C) shows an optical micrograph in cross section of
spheroidized graphite in the course of spheroidizing treatment.
[0038] FIG. 6 shows a SEM observation image of a natural graphite
particle after spheroidizing treatment.
[0039] FIG. 7 shows a SEM observation image of a modified natural
graphite particle according to the present invention obtained by
carrying out smoothening treatment after spheroidizing
treatment.
MODES FOR CARRYING OUT THE INVENTION
[0040] Modified natural graphite particles and a method for their
manufacture will be explained below.
1. Natural Graphite Particles
[0041] Natural graphite particles (referred to below as raw
graphite material), which are raw materials of modified natural
graphite particles according to the present invention, are
particles of flake-shaped graphite (specifically the
below-described flake graphite and vein graphite) which has not
undergone any modification treatment or heat treatment. Modified
natural graphite particles can be manufactured by subjecting the
natural graphite particles to the below-described shape-modifying
treatment, for example.
[0042] Based on its outer appearance and properties, natural
graphite is classified as flake graphite, vein graphite (also
called massive graphite), and amorphous graphite. Flake graphite
and vein graphite are nearly completely crystalline, while
amorphous graphite has a lower crystallinity The quality of natural
graphite is determined primarily by the place of origin and the
vein from which it is derived. Flake graphite is produced in
locations such as Madagascar, China, Brazil, the Ukraine, Canada,
Vietnam, and Australia. Vein graphite is produced primarily in Sri
Lanka. Amorphous graphite is produced in locations such as the
Korean Peninsula, China, and Mexico.
[0043] Since a high capacity is demanded of modified natural
graphite particles, flake graphite and vein graphite, which both
have a high crystallinity, are suitable as the raw graphite. One
criterion for assessing the crystallinity of graphite particles is
the absolute specific gravity thereof. The raw graphite preferably
has an absolute specific gravity of at least 2.25 g/cm.sup.3. The
absolute specific gravity of graphite is not significantly affected
by mechanical modification treatment, so the resulting modified
natural graphite particles also preferably have an absolute
specific gravity of at least 2.25 g/cm.sup.3.
[0044] The shape and particle size of the raw graphite are not
critical. The raw graphite may be prepared by mixing two or more
types of graphite having different places of origin or belonging to
different classes.
[0045] In the present invention, the particle shape is assessed in
terms of circularity, which is used as an index of sphericity. The
circularity is determined by the following equation dealing with
the projected shape of a particle:
Circularity=(Circumference of a circle having the same area as a
projected shape of a particle)/(Circumference of the projected
shape of the particle).
[0046] When the projected shape makes a perfect circle, its
circularity is 1. Therefore, the maximum value of circularity is 1.
A particle having a circularity of 1 is expected to have a
particularly high degree of sphericity when it is assessed three
dimensionally for sphericity. Therefore, it can be said that the
higher (the closer to 1) is the circularity of a particle, the
higher is its sphericity. The projected shape of particles can be
obtained by an image observed with an optical microscope or a
scanning electron microscope.
[0047] For example, when flake graphite is used as the raw
graphite, the circularity of the raw graphite is usually in the
vicinity of 0.85 and seldom exceeds 0.90. The raw graphite may be
one having such a low circularity, and the circularity of the raw
graphite is not critical. In the present invention, the raw
graphite becomes modified natural graphite particles having a
circularity of at least 0.92 after it has been subjected to
modification treatment.
[0048] In the present invention, the size of graphite particles is
assessed in terms of the mean particle diameter, which is the
median diameter of particles in a volume-based particle size
distribution determined by light scattering diffractometry. Such a
particle size distribution can be measured using a laser
diffraction and scattering particle size analyzer (LA-910)
manufactured by Horiba, Ltd., for example.
[0049] If the raw graphite has an excessively large particle size,
the efficiency of spheroidizing treatment which is carried out in
order to obtain modified natural graphite particles is decreased.
Therefore, the raw graphite preferably has a mean particle diameter
of at most 5 mm and particularly preferably at most 200 .mu.m. On
the other hand, if the mean particle diameter of the raw graphite
is excessively small, it may be difficult to control the shape by
shape modifying treatment with some specific means, or there may be
a concern that the raw graphite causes air pollution with dust.
Therefore, the mean particle diameter of the raw graphite is
preferably at least 3 .mu.m and particularly preferably at least 5
.mu.m.
2. Modified Natural Graphite Particles
[0050] Modified natural graphite particles according to the present
invention have a circularity of at least 0.92 and an incident angle
dependence S.sub.60/0 of the peak intensity ratio (details of which
will described below) of at least 0.5 and at most 0.7, with the
peak intensity ratio being determined by measurement of C K-edge
X-ray absorption spectra.
[0051] The properties of the modified natural graphite particles
according to the present invention will be described below in
detail.
[0052] (1) Circularity
[0053] The modified natural graphite particles according to the
present invention have a circularity of at least 0.92. If the
circularity is less than 0.92, the graphite particles take a
flattened shape having a large aspect ratio and easily cause
problems such as undesirable orientation during coating and a
decrease in the capacity of a battery. The upper limit of the
circularity is 1.0 which is the circularity when the particle shape
is perfectly spherical. The circularity is preferably at least
0.93.
[0054] (2) Peak Intensity Ratio in C K-Edge X-Ray Absorption
Spectra
[0055] The modified natural graphite particles according to the
present invention have an incident angle dependence S.sub.60/0 of
the peak intensity ratio defined by the following equation of at
least 0.5 and at most 0.7, with the peak intensity ratio being
determined by measurement of C K-edge X-ray absorption spectra
using synchrotron radiation as an excitation source:
S.sub.60/0=I.sub.60/I.sub.0
where
[0056] I.sub.60=B.sub.60/A.sub.60
[0057] I.sub.0=B.sub.0/A.sub.0
[0058] A.sub.60: the intensity of the absorption peak assigned to
the transition from C-1s level to .pi.* level (i.e., antibonding
orbital of sp2 bond: --C.dbd.C--) in a C K-edge X-ray absorption
spectrum of particles when measured with synchrotron radiation
having an angle of incidence of 60.degree.;
[0059] B.sub.60: the intensity of the absorption peak assigned to
the transition from C-1s level to .sigma.* level (i.e., antibonding
orbital of sp3 bond: --C--C--) in a C K-edge X-ray absorption
spectrum of particles when measured with synchrotron radiation
having an angle of incidence of 60.degree.;
[0060] A.sub.0: the intensity of the absorption peak assigned to
the transition from C-1s level to .pi.* level in a C K-edge X-ray
absorption spectrum of particles when measured with synchrotron
radiation having an angle of incidence of 0.degree.; and
[0061] B.sub.0: the intensity of the absorption peak assigned to
the transition from C-1s level to .sigma.* level in a C K-edge
X-ray absorption spectrum of particles when measured with
synchrotron radiation having an angle of incidence of
0.degree..
[0062] The reason why the ratio S.sub.60/0 is limited to a
particular range will be described below in detail.
[0063] i) Measuring Method
[0064] The C K-edge (carbon K-edge) X-ray absorption spectrum,
which is also referred to as the C K-edge NEXAFS (Near Edge X-ray
Absorption Fine Structure) spectrum, is observed when electrons
(i.e., K-shell core electrons) which exist at the inner shell level
(1s orbital) of a carbon atom which are in an occupied state are
excited to various empty levels which are in an unoccupied state by
absorbing the energy of irradiated X-rays.
[0065] The principle of X-ray absorption spectroscopy is shown in
FIG. 1 compared to X-ray photoelectron spectroscopy (XPS).
[0066] In the measurement of C K-edge NEXAFS spectra, synchrotron
radiation is used as an excitation source. This is because an
energy-variable light source in the soft X-ray region (280 eV-320
eV) is necessary in order to observe the electronic transition from
the inner shell level of carbon having a bond energy of 283.8 eV to
to various empty levels and because the quantitativeness of the
S.sub.60/0 ratio is premised on the excitation source having a high
degree of linear polarization.
[0067] The empty levels to which electrons at the inner shell level
are excited include the .pi.* level which is assigned to an
antibonding orbital of a sp2 bond and which reflects the
crystallinity (such as basal planes and degree of orientation) of
natural graphite, the .sigma.* level which is assigned to an
antibonding orbital of a sp3 bond and which reflects disorders of
the crystallinity (such as edge planes and the degree of
non-orientation), and an empty level assigned to an antibonding
orbital such as a C--H bond or a C--O bond. In graphite having a
crystal structure comprising stacked hexagonal networks made by sp2
bonds, a plane having the surface of a hexagonal network (an AB
plane discussed later) is a basal plane, while a plane in which
edges of hexagonal networks appear is an edge plane. In an edge
plane, carbon atoms often have sp3 bonds.
[0068] A C K-edge NEXAFS spectrum reflects a local structure in the
vicinity of a carbon atom including the excited inner-shell
electrons, and it also reflects only the surface structure of a
graphite particle which is measured because the depth of escape of
electrons which are discharged into a vacuum from a solid by the
irradiated light is on the order of 10 nm. Accordingly, C K-edge
NEXAFS spectra can be used to measure the crystallinity (degree of
orientation) of graphite which is present on the surface of a
modified natural graphite particle, thereby evaluating the
roughness of graphite surface.
[0069] There is no particular limitation on a method of fixing
modified natural graphite particles which are to be measured to a
sample table. In order to avoid a change of the surface properties
of a graphite particle due to application of an excess load, it is
preferable to employ a method in which the particles are supported
on a copper substrate with an In (indium) foil or a carbon
tape.
[0070] For the measurement of a C K-edge NEXAFS spectrum according
to the present invention, a sample is irradiated with synchrotron
radiation having an angle of incidence which is fixed with respect
to the sample. The measurement is carried out by the total electron
yield method in which the electric current which is flowing into
the sample in order to supplement the photoelectrons discharged
from the sample is measured while the energy of synchrotron
radiation which is irradiated is scanned from 280 eV to 320 eV. The
basic setup for this measuring method is shown in FIG. 2.
[0071] II) Quantitative Assessment of the Degree of Orientation of
Surface Graphite Crystal by S.sub.60/0
[0072] As discussed below, it is possible to quantitatively assess
the degree of orientation of the graphite crystal of a modified
natural graphite particle in the vicinity of the surface (referred
to below as the surface graphite crystal) by measuring the value of
S.sub.60/0.
[0073] Due to the high linear polarization of synchrotron
radiation, when the direction of incidence of synchrotron radiation
is parallel with the direction of the bonding axis of sp2
(--C.dbd.C--) bonds of a surface graphite crystal, the intensity of
an absorption peak assigned to the transition from C-1s level to
.pi.* level increases, while it decreases when the two directions
are perpendicular to each other.
[0074] As a result, in the case of a sample such as highly oriented
pyrolytic graphite (HOPG or monocrystalline graphite) in which the
sp2 bond-forming graphite crystal is highly oriented in the
vicinity of the surface, the spectrum is greatly changed by
changing the angle of incidence of synchrotron radiation with
respect to the sample, but in the case of a sample such as a
vapor-deposited carbon film (amorphous) in which the degree of
orientation of the sp2 bond-forming carbon material is low in the
vicinity of the surface, the spectrum shape is not changed
significantly by changing the angle of incidence of synchrotron
radiation with respect to the sample.
[0075] FIGS. 3(A) and 3(B) show C K-edge NEXAFS spectra when
synchrotron radiation is applied with different angles of incidence
(0.degree., 30.degree., and 60.degree.) with respect to carbon
wherein FIG. 3(A) is a case in which the carbon is HOPG (Highly
Oriented Pyrolytic Graphite), which is monocrystalline, and FIG.
3(B) is a case in which the carbon is a vapor-deposited carbon film
(with a thickness of 10 nm), which is amorphous. As shown in FIG.
3(A), with HOPG which is a monocrystal, when the angle of incidence
is increased from 0.degree. to 60.degree., the intensity of an
absorption peak A which is assigned to the transition from C-1s
level to .pi.* level increases, while the intensity of an
absorption peak B which is assigned to the transition from C-1s
level to .sigma.* level decreases. Therefore, the profile of a C
K-edge NEXAFS spectrum of HOPG greatly changes depending on the
angle of incidence. In contrast, as shown in FIG. 3(B), the shape
of a C K-edge NEXAFS spectrum of a vapor-deposited carbon film,
which is amorphous, scarcely depends on the angle of incidence, so
the profile does not significantly change even if the angle of
incidence is changed.
[0076] Accordingly, if the ratio I (=A/B) of the absorption peak
intensity A to the absorption peak B varies depending on the angle
of incidence as a result of measurement of C K-edge NEXAFS spectra
of a certain graphitic material with different angles of incidence,
it is thought that graphite crystals of the material which are
present in the vicinity of the surface are arranged with regular
alignment, namely with a high degree of orientation. On the
contrary, when there is no dependence of the ratio I on the angle
of incidence, graphite crystals of the material which are present
in the vicinity of the surface are arranged irregularly with a low
degree of orientation. Thus, by quantifying the dependence of the
ratio I of absorption intensity A to absorption intensity B on the
angle of incidence of radiation, it becomes possible to
quantitatively assess the degree of orientation of graphite
crystals of a graphitic material which are present in the vicinity
of the surface.
[0077] Therefore, in accordance with the present invention, the
dependence of the ratio of peak intensity on the angle of incidence
S.sub.60/0 (=I.sub.60/I.sub.0) which is derived from the ratios
I.sub.60 and I.sub.0 of absorption intensity A to absorption
intensity B at two angles of incidence, 60.degree. and 0.degree.,
respectively is used to quantitatively assess the degree of
orientation of graphite crystals which exist in the vicinity of the
surface of a graphitic material. FIG. 4 is a diagram explaining a
method for quantitatively assessing the degree of orientation of
surface graphite crystals according to the present invention by
taking a case in which the sample is HOPG as an example.
[0078] When the value of S.sub.60/0 is in the vicinity of 1, the
degree of orientation of surface graphite crystals is low, and as
the value of S.sub.60/0 is closer to 0 (FIG. 4), the degree of
orientation of surface graphite crystals is higher.
[0079] When sample particles are supported using indium foil or
carbon tape in order to determine the value of S.sub.60/0, the C
K-edge NEXAFS spectrum of the support is measured as a blank
spectrum, and the intensity of the C K-edge NEXAFS spectrum
obtained by measurement of the sample particles is calibrated using
the blank spectrum to calculate the intensities of absorption peaks
due to the individual transitions.
[0080] iii) Relationship Between S.sub.60/0 and the Bonding
Strength of a Negative Electrode Mixture
[0081] Modified natural graphite particles according to the present
invention are obtained from a flake-shaped raw graphite material
and have a circularity of at least 0.92. The raw graphite material
is subjected to treatment (referred to below as spheroidizing
treatment) in order to make the shape of the entire raw graphite
material close to spherical and thereby obtain the above-mentioned
circularity. Specific examples of the spheroidizing treatment
include the treatments disclosed in Patent Documents 2-4.
[0082] The flake-shaped graphite used as a raw material is a
crystal composed of a great number of hexagonal network planes (AB
planes) stacked on one another, each plane having carbon atoms
which regularly form a planar network, and the crystal has a
thickness in the C direction which is perpendicular to the AB
planes. Peeling between the AB planes easily takes place because
the bonding force between the stacked AB planes (van der Waals
force) is much smaller than the bonding force of the AB planes in
the in-plane direction. Therefore, the thickness of the stacked
planes is thin compared to the extent of the AB planes, thereby
providing a flake shape as a whole.
[0083] As shown in FIGS. 5(A) to 5(C), when the flake-shaped raw
graphite material undergoes spheroidizing treatment, the raw
graphite material which originally had a nearly planar form is
folded, incorporated into another particle when the particle is
folded, or adheres to the surface of another particle.
[0084] Therefore, in a macroscopic view, the surface of a graphite
particle obtained by spheroidizing treatment of a raw graphite
material (referred to below as spheroidized graphite particle) is
mostly covered by the surface (an AB plane) of the raw graphite
material which was planar. Accordingly, it is thought that the
surface of the spheroidized graphite particle is predominantly
constituted by AB planes. However, as shown in FIG. 6, when a
spheroidized graphite particle is observed with magnification,
there are many irregularities because an end face of a folded
particle or an end face of an attached particle, namely, an edge
plane is exposed on the surface of the particle. Furthermore, in a
microscopic view, the impact force applied during the spheroidizing
treatment causes portions of the AB planes to be peeled off and
folded, resulting in the formation of portions in which an edge
plane is exposed on the surface of the particle.
[0085] Such surface irregularities cause the surface of a
spheroidized graphite particle to have a rough surface. Therefore,
the bonding axes of sp2 bonds in graphite crystals existing in the
vicinity of the surface of a spheroidized graphite particle have
random directions. As a result, a spheroidized graphite particle
has a circularity of at least 0.92, but its value of S.sub.60/0 is
in the vicinity of 1.
[0086] If a spheroidized graphite particle having surface
properties which are made rough by spheroidizing treatment, namely,
a spheroidized graphite particle having a circularity of at least
0.92 and a value of S.sub.60/0 in the vicinity of 1 is used as a
negative electrode active material, only the protruding portions of
the rough surface of the spheroidized graphite particle can form
contact portions between particles of the negative electrode active
material or between a particle of the negative electrode active
material and a current collector. A binder which constitutes a
negative electrode mixture along with a negative electrode active
material essentially serves to bond particles of the negative
electrode active material to each other or the negative electrode
active material to the current collector by being supplied to the
above-described contact portions. Therefore, use of the
above-described spheroidized graphite particle as a negative
electrode active material results in the formation of a negative
electrode mixture which has low bonding properties and which is
easily peeled off from a current collector.
[0087] In contrast, a modified natural graphite particle according
to the present invention has a circularity of at least 0.92 and a
value of S.sub.60/0 of 0.7 or less, indicating that although it has
undergone spheroidizing treatment, the particle has surface
graphite crystals which are oriented to some extent. In portions of
the surface graphite particles which are oriented, the AB planes
are very flat with minimized defects such as folding, so a negative
electrode material made of this graphite particle can form a
relatively large contact portion with an adjacent negative
electrode material or with a current collector.
[0088] As a result, use of a modified natural graphite particle
according to the present invention as a negative electrode active
material makes it possible to obtain a negative electrode mixture
which has good bonding properties and which is difficult to peel
off from a current collector.
[0089] There is no particular lower limit on the value of
S.sub.60/0 of a modified natural graphite particle according to the
present invention as long as it has a circularity of at least 0.92.
However, a practical lower limit is 0.5. If the value of S.sub.60/0
is less than 0.5, it is quite difficult in actuality to make the
circularity at least 0.92.
[0090] (3) Tap Density
[0091] A modified natural graphite particle according to the
present invention preferably has a tap density of at least 1.0
g/cm.sup.3 and at most 1.4 g/cm.sup.3 as measured by tapping 180
times using a vessel having a capacity of 100 cm.sup.3.
[0092] By having a tap density of at least 1.0 g/cm.sup.3, the
packing density of a negative electrode active electrode in a
negative plate is increased. The tap density is preferably at least
1.05 g/cm.sup.3. Graphite particles manufactured by subjecting a
raw graphite material to only spheroidizing have rough surfaces, so
it is not easy for them to have an increased tap density. The
higher the tap density the better. However, in actuality, the upper
limit on the tap density is 1.4 g/cm.sup.3.
[0093] (4) Linseed Oil Absorption
[0094] A modified natural graphite particle according to the
present invention preferably has linseed oil absorption of at least
20 cm.sup.3/100 g and at most 50 cm.sup.3/100 g as measured by an
absorption tester generally in accordance with the method for
measuring an absorbed oil amount defined in JIS K 6217-4:2008.
[0095] Graphite particles manufactured by subjecting a raw graphite
material to only spheroidizing have excessively rough surfaces, so
they tend to show increased linseed oil absorption. Excessively
high linseed oil absorption results in a decrease in the efficiency
of utilization of a binder, thereby making it difficult to increase
capacity. Therefore, the linseed oil absorption is preferably at
most 50 cm.sup.3/100 g. The lower the linseed oil absorption the
better. However, in actuality, the lower limit on the linseed oil
absorption is 20 cm.sup.3/100 g.
[0096] (5) Coating Layer
[0097] A modified natural graphite particle according to the
present invention having the above-described properties may be a
carbon-deposited graphite particle having a carbonaceous material
adhering to its surface. As a result, it has an improved battery
performance.
[0098] The term carbonaceous material as used herein refers to a
material which predominantly comprises carbon. Its structure is not
critical. The carbonaceous material may either adhere to a portion
of the surface of a modified natural graphite particle or adhere
such that substantially the entire surface of the particle is
coated with the carbonaceous material.
[0099] Preferably, the carbonaceous material has a lower
crystallinity than the modified natural graphite particle which is
a core to be coated and/or a higher proportion of sp3 bonds in all
the carbon-carbon bonds. Such a carbonaceous material has a higher
bulk hardness than a graphite particle. Therefore, the presence of
the carbonaceous material by adhesion on the surface of the
modified natural graphite particle increases the hardness of the
entire particle. As a result, in the process for manufacturing a
negative plate, particularly in its compression step, there is a
minimized possibility of charge acceptance decreasing due to the
formation of closed pores inside an electrode comprising a negative
electrode active material. In addition, as can be seen from the
below-described examples, adhesion of carbon results in a decrease
of the surface area of a graphite particle, so the reactivity of
the particle with an electrolytic solution is suppressed.
Accordingly, a negative plate comprising such carbon-deposited
graphite particles as an active material is improved with respect
to charge-discharge efficiency and battery capacity.
[0100] An example of a carbonaceous material having a lower
crystallinity than a modified natural graphite particle which is a
core is turbostratic carbon. The term turbostratic carbon as used
herein means a carbon material in which carbon atoms form a layer
structure stacked parallel to the direction of a hexagonal network
plane but their crystalline regularity in the three-dimensional
direction cannot be measured.
[0101] An example of a carbonaceous material having a lower
crystallinity and a higher proportion of sp3 bonds than a modified
natural graphite particle which is a core is amorphous carbon. The
term amorphous carbon as used herein means a carbon material which
has a short range order (an order from several atoms to ten-some
atoms) but does not have a long range order (an order from several
hundred to several thousand atoms).
[0102] There are no particular limitations on a method of adhering
or coating a carbonaceous material on the surface of a graphite
particle serving as a core. Typical examples are a surface
treatment method and a deposition method using vacuum film forming
techniques. A surface treatment method is a method in which an
organic compound such as pitch is previously adhered to or coated
on at least a portion of the surface of graphite powder and heat
treatment is then performed to carbonize the organic compound. This
method results in the formation of a carbonaceous material made of
turbostratic carbon. By a vacuum film forming technique, a
carbonaceous material made of amorphous carbon can be deposited on
the surface of a core.
3. Method of Manufacturing a Modified Natural Graphite Particle
[0103] A modified natural graphite particle according to the
present invention may be manufactured by any manufacturing method
as long as the particle has the above-described properties. One
method capable of stably and efficiently producing a modified
natural graphite particle having the above-described properties is
described below, The conditions in each treatment step can be
suitably adjusted so that a modified natural graphite particle
having the above-described properties is obtained.
[0104] (1) Spheroidizing Treatment
[0105] A raw natural graphite particle can be spheroidized using an
impact-type pulverizing device such as a jet mill or a pin mill. As
shown in FIGS. 5(A) to 5(C), this type of device causes raw natural
graphite particles to impinge against pins or the like at a high
velocity, thereby resulting in a decrease in the aspect ratio of
the graphite particles due to bending of the stacked AB planes or
adhesion of other particles.
[0106] However, as shown in FIG. 6, numerous minute surface
irregularities (depressions and protrusions) are present on the
surface of a graphite particle which has undergone spheroidizing
treatment (spheroidized graphite particle) due to turning-up or
peeling of a portion of the stacked AB planes or exposure of an end
face on the surface of the particle. As a result, the particle has
a rough surface. Spheroidizing is carried out such that graphite
particles having a circularity of at least 0.92 are obtained.
[0107] (2) Smoothening Treatment
[0108] Mechanical grinding treatment is applied to the spheroidized
particle obtained above, thereby making its surface smoothen and
making it possible to obtain a modified natural graphite particle
according to the present invention.
[0109] Mechanical grinding treatment is carried out in order to
round angular portions of a particle and smoothen minute surface
irregularities of the particle. For this purpose, for example, an
apparatus which repeatedly imparts mechanical actions including
interactions between particles such as compression, friction, and
shear to particles can be used. A powder processing apparatus
manufactured by Hosokawa Micron Corporation (Circulating Mechano
Fusion System AMS-Lab), a Theta Composer manufactured by Tokuju
Corporation, or the like can be used as an apparatus for carrying
out mechanical grinding.
[0110] With such an apparatus, graphite particles are allowed to
pass through a gap formed by two solid bodies which are in movement
relative to each other with a small spacing therebetween (for
example, a rotor and an inner piece), thereby imparting a strong
sliding force in the in-plane direction to the surface of the
graphite particles. Therefore, when the graphite particles pass
through the gap, the crystals in the location of the particles
which undergo sliding are oriented in the sliding direction.
Namely, exposed folded end faces of the particles or end faces of
adhered particles are covered by the AB planes as a result of
sliding occurring between AB plane layers. In addition, portions of
the AB planes in which an edge portion faces toward the surface of
the particle due to peeling or folding which are present in some
places of the AB planes are compressed and oriented.
[0111] In this manner, as shown in FIG. 7, the surface of a
spheroidized graphite particle is smoothened by smoothening
treatment such as mechanical grinding treatment, and the resulting
modified natural graphite particle has surface graphite crystals
with an increased degree of orientation. As a result, it is
possible to obtain a modified natural graphite particle according
to the present invention which has a circularity of at least 0.92
and a value for S.sub.60/0 of at most 0.7.
[0112] A modified natural graphite particle according to the
present invention and a carbon-deposited graphite particle having a
core of the modified natural graphite particle can be used as an
active material to manufacture a negative plate for a non-aqueous
electrolyte secondary battery. There are no particular limitations
on a binder and current collector used in the manufacture of a
negative electrode, and each may be one which has conventionally
been used. According to the present invention, because the surface
of graphite particles used as an active material is smoothened and
the contact area of the graphite particles with each other or with
a current collector is increased, it is possible to decrease the
amount of a binder compared to the prior art, thereby making it
possible to manufacture an electrode having a higher density and a
higher capacity.
EXAMPLES
Examples 1-4 and Comparative Examples 1-4
(1) Manufacture of Modified Natural Graphite Particles
[0113] Raw natural graphite particles (flake graphite produced in
China with an absolute specific gravity of 2.26 g/cm.sup.3) were
subjected to spheroidizing treatment using a pulverizer
manufactured by Hosokawa Micron Corporation (ACM Pulverizer, Model
ACM-10A). The treatment was repeatedly performed 15 times.
Subsequently, dust was removed by air classification. By carrying
out the spheroidizing treatment with suitably different rotational
speeds in the pulverization and classification, four types of
spheroidized graphite particles having different particle sizes
which are shown as Comparative Examples 1-4 in Table 1 were
obtained.
[0114] Some of these graphite particles were further subjected to
smoothening treatment using a Mechano Fusion System manufactured by
Hosokawa Micron Corporation (AMS-Lab). The conditions for treatment
were as follows:
[0115] Input amount of particles: 600 grams
[0116] Gap between rotor and inner piece: 5 mm
[0117] Rotational speed: 2600 rpm
[0118] Length of treatment: 15 minutes.
[0119] By the smoothening treatment, modified natural graphite
particles shown as Examples 1-4 in Table 1 were obtained.
[0120] The properties (S.sub.60/0, mean particle diameter,
circularity, specific surface area, tap density, and linseed oil
absorption) of each of the resulting spheroidized graphite
particles of Comparative Examples 1-4 and modified natural graphite
particles of Examples 1-4 were determined by the methods described
below, and the results are shown in Table 1. The absolute specific
density of these graphite particles is not shown in Table 1, but
was the same as that of the raw graphite particles (2.26
g/cm.sup.3).
(2) Preparation of Negative Plates
[0121] A negative electrode active material consisting of
spheroidized graphite particles or modified natural graphite
particles obtained by the above-described method was mixed with a
binder to prepare two types of negative electrode mixtures
(negative electrode mixtures 1 and 2).
[0122] [Negative Electrode Mixture 1]
[0123] The graphite particles were mixed with a binder consisting
of a styrene-butadiene rubber (SBR) and sodium
carboxymethylcellulose (CMC) to prepare a negative electrode
mixture. The proportions (mass ratio) of the components in the
negative electrode mixture were as follows:
[0124] negative electrode active material:SBR:CMC=98:1:1.
[0125] [Negative Electrode Mixture 2]
[0126] The graphite particles were mixed with a polyvinylidene
fluoride (PVdF) as a binder. The proportions (mass ratio) of the
components in the negative electrode mixture were as follows:
[0127] negative electrode active material:PVdF=9:1.
[0128] Each negative electrode mixture was applied to an
electrolytic copper foil (with a thickness of 17 .mu.m) serving as
a current collector, then dried (for 20 minutes at 75.degree. C.
for negative electrode mixture 1 or for 20 minutes at 100.degree.
C. for negative electrode mixture 2), and compressed by uniaxial
pressing to obtain a negative plate. The negative electrode mixture
layer in each of the resulting negative plates was 9 mg/cm.sup.2
and its density was 1.6 g/cm.sup.3. The peel strength of each
negative plate was measured by the below-described method, and the
result is shown in to Table 1.
(3) Measurement Methods
[0129] i) S.sub.60/0
[0130] Measurement of a C K-edge NEXAFS spectrum was carried out
using Beamlines BL7B and BL9 in the NewSUBARU synchrotron radiation
facility installed by Hyogo prefecture in the Spring-8 site and
managed by the Laboratory of Advanced Science and Technology for
Industry of the University of Hyogo. The excitation light source
was synchrotron radiation which was discharged when electrons
accumulated in a storage ring with an acceleration voltage of
1.0-1.5 GeV and a storage ring current of 80-350 mA were passed
along a serpentine path through an inserted light source referred
to as an undulator. Using a C K-edge NEXAFS spectrum measuring
device installed in the Beamlines BL7B and BL9, C K-edge NEXAFS
spectra were measured for the graphite particles of each of the
Examples and Comparative Examples. The value of S.sub.60/0 was
calculated from the spectrum profiles obtained at incident angles
of 0.degree. and 60.degree.. The details of the principle of
measurement and measuring method are described above. The support
used to support sample particles was an In foil.
[0131] ii) Mean Particle Diameter (Indicated as d50 in Table 1)
[0132] A particle size distribution on a volume basis of the
graphite particles in each example was measured by the light
diffraction/dispersion method using a laser diffraction/dispersion
particle size analyzer (LA-910) manufactured by Horiba, Ltd. The
median diameter in the resulting particle size distribution was
taken as the mean particle diameter of the graphite particles.
[0133] iii) Circularity
[0134] The circularity of the graphite particles in each example
was measured using a flow particle image analyzer (FPIA-2100)
manufactured by Sysmex Corporation. Specifically, at least 5000 of
the graphite particles were the subject of measurement. A flat
sample stream in the form of a dispersion of the particles in a
dispersion medium formed by addition of polyoxyethylene sorbitan
monolaurate as a surfactant to deionized water was photographed,
and the resulting image of the particles was subjected to image
analysis to determine the circularity.
[0135] iv) Specific Surface Area
[0136] The specific surface area was determined by the BET 1-point
method using a Quantasorb manufactured by Yuasa Ionics Co.,
Ltd.
[0137] v) Tap Density
[0138] Using a Model PT-N Powder Tester (registered trademark)
manufactured by Hosokawa Micron Corporation, tapping of the
graphite particles in each example was carried out 180 times in a
vessel having a capacity of 100 cm.sup.3, and the apparent specific
density was then measured and taken as the tap density.
[0139] vi) Linseed Oil Absorption
[0140] The linseed oil absorption of the graphite particles in each
example was measured generally in accordance with the oil
absorption measuring method defined in JIS K 6217-4:2008 using a
Model S-410 Absorption Tester manufactured by Asahisouken Co., Ltd.
Specifically, linseed oil was added at a rate of 4 cm.sup.3/min to
graphite particles which were being stirred with two blades. The
change in the viscosity at this time was detected with a torque
sensor and converted into torque. The added amount of linseed oil
corresponding to 100% of the maximum generated torque was converted
to the amount per 100 g of graphite particles to determine the
linseed oil absorption.
[0141] vii) Peel Strength
[0142] The peel strength was determined generally in accordance
with JIS C 6481. Specifically, a strip of a negative plate cut to a
width of 15 mm was placed on a table and secured thereto with
double-sided tape (NW-K15 manufactured by Nichiban Co. Ltd.) so
that the negative electrode mixture was facing down. The current
collector forming the upper surface of the secured negative plate
was pulled vertically with respect to the top surface of the table
at a speed of 50 mm/min to peel off the strip by a length of 50 mm.
The load for peeling at this time was continuously measured, and
the smallest value of the load was recorded as the peel strength
(N/m).
TABLE-US-00001 TABLE 1 Oil absorp- Mixture 1 Mixture 2 Tap tion
Peel Peel d50 SSA* density Circu- (mL/ strength strength S.sub.60/0
(.mu.m) (m.sup.2/g) (g/cm.sup.3) larity 100 g) (N/m) Ratio.sup.1)
(N/m) Ratio.sup.1) Example 1 0.51 14.7 7.5 1.05 0.93 46.0 27 1.80
15 7.50 Example 2 0.65 18.2 6.0 1.09 0.94 42.1 31 1.72 41 2.73
Example 3 0.68 21.0 4.9 1.10 0.94 41.0 35 1.84 45 2.65 Example 4
0.55 24.5 4.3 1.06 0.93 46.5 32 1.88 38 2.11 Comp. 1 0.79 14.7 7.5
1.00 0.93 53.0 15 -- 2 -- Comp. 2 0.84 18.2 6.0 1.05 0.94 50.2 18
-- 15 -- Comp. 3 0.88 21.1 4.9 1.05 0.94 50.1 19 -- 17 -- Comp. 4
0.75 24.5 4.2 1.01 0.93 54.8 17 -- 18 -- *SSA = Specific surface
area; **Comp. = Comparative Example .sup.1)Strength ratio of peel
strength after smoothening/before smoothening
[0143] The strength ratio in Table 1 is a ratio of peel strength
after smoothening treatment to peel strength before smoothening
treatment, and specifically it is determined by dividing the peel
strength in Example 1 by the peel strength in Comparative Example,
for example.
[0144] As can be seen from Table 1, the graphite particles of
Comparative Examples 1-4 which were obtained by spheroidizing
treatment had a circularity of at least 0.92, but they had a large
value of S.sub.60/0 which was in the range of 0.75-0.88 with their
linseed oil absorption exceeding 50 cm.sup.3 per 100 grams. In
contrast, the graphite particles of Examples 1-4 which further
underwent smoothening treatment had a decreased value of S.sub.60/0
in a range of 0.51-0.68 and their linseed oil absorption also
decreased to less than 50 cm.sup.3 per 100 grams. Comparing
corresponding Examples and Comparative Examples (e.g., Example 1
and Comparative Example 1), the tap density in the Examples was
higher than that in the Comparative Example.
[0145] Compared to the spheroidized graphite particles in
Comparative Examples 1-4, the modified natural graphite particles
in Examples 1-4 according to the present invention had a peel
strength which was increased by a factor of 1.72 to 1.88 for
Mixture 1 or by a factor of 2.11 to 7.50 for Mixture 2, and it was
confirmed that the peel strength was significantly improved.
Example 5 and Comparative Example 5
[0146] The graphite particles obtained in each of Example 2 and
Comparative Example 2 were mixed with a coal-derived pitch powder
having a mean particle diameter of 15 .mu.m in a proportion of 20
mass % based on the mass of the graphite particles. The mixture was
heat-treated for 1 hour at 1000.degree. C. in a nitrogen stream to
obtain carbon-deposited graphite particles having turbostratic
carbon adhering to the surface of the graphite particles. The mean
particle diameter, specific surface area, tap density, and linseed
oil absorption of the resulting carbon-deposited graphite particles
were determined in the same manner as in Examples 1-4. The results
are shown in Table 2.
[0147] The carbon-deposited graphite particles which were obtained
in this manner were used as a negative electrode active material
and were mixed with PVdF at a mass ratio of 95:5 to prepare
negative electrode mixtures. The negative electrode mixtures were
used to manufacture negative plates in the same manner as in
Examples 1-4. The peel strength of each of the resulting negative
plates was measured in the same manner as in Example 1-4, and the
results are shown in Table 2 along with the value of the strength
ratio.
TABLE-US-00002 TABLE 2 Strength Specific Oil ratio (After/ surface
Tap absorption Peel Before d50 area density (mL/ strength
smoothening (.mu.m) (m.sup.2/g) (g/cm.sup.3) 100 g) (N/m)
treatment) Example 5 18.5 0.6 1.30 27.0 89.0 2.28 Comp. 18.4 0.6
1.21 36.1 39.0 -- Ex. 5
[0148] As can be seen from Table 2, in Example 5 in which the core
was a modified natural graphite particle according to the present
invention, the peel strength was 2.28 times as high as that in
Comparative Example 5 in which the core was a spheroidized graphite
particle. It can be seen by comparison between Tables 1 and 2 that
by carrying out the carbon deposition treatment, a graphite
particle had a significantly decreased specific surface area and an
increased tap density, but its mean particle diameter was little
increased. In these examples, because the coal-derived pitch powder
used for the formation of a carbonaceous material was used in a
relatively large amount, it is thought that nearly all the surfaces
of the graphite particles were coated with a carbonaceous material
(turbostratic carbon). As a result, fine surface irregularities
were filled, leading to a significant decrease in specific surface
area.
Example 6 and Comparative Example 6
[0149] The graphite particles obtained in each of Example 3 and
Comparative Example 3 were mixed with a coal-derived pitch powder
having a mean particle diameter of 15 .mu.m in a proportion of 2
mass % based on the mass of the graphite particles. The mixture was
heat-treated for 1 hour at 1000.degree. C. in a nitrogen stream to
obtain carbon-deposited graphite particles having turbostratic
carbon adhering to the surface of the graphite particles. The mean
particle diameter, specific surface area, tap density, and linseed
oil absorption of the resulting carbon-deposited graphite particles
were determined in the same manner as in Examples 1-4. The results
are shown in Table 3.
[0150] The carbon-deposited graphite particles which were obtained
in this manner were used as a negative electrode active material
and were mixed with SBR and CMC at a mass ratio of 98:1:1 to
prepare negative electrode mixtures. The negative electrode
mixtures were used to manufacture negative plates in the same
manner as in Examples 1-4. The peel strength of each of the
resulting negative plates was measured in the same manner as in
Examples 1-4, and the results are shown in Table 3 along with the
value of the strength ratio.
TABLE-US-00003 TABLE 3 Strength Specific Oil ratio (After/ surface
Tap absorption Peel Before d50 area density (mL/ strength
smoothening (.mu.m) (m.sup.2/g) (g/cm.sup.3) 100 g) (N/m)
treatment) Example 6 21.1 3.6 1.20 37.8 32.9 1.89 Comp. 21.1 3.6
1.11 45.1 17.4 -- Ex. 6
[0151] As can be seen from Table 3, in Example 6 in which the core
was a modified natural graphite particle according to the present
invention, the peel strength was 1.89 times as high as that in
Comparative Example 6 in which the core was a spheroidized graphite
particle. In these examples, because the coal-derived pitch powder
used for the formation of a carbonaceous material was used in a
small amount of 2 mass % of the graphite particles, it is thought
that only a portion of the surfaces of the graphite particles was
coated with a carbonaceous material in the form of turbostratic
carbon. Even in this case, the specific surface area of the
graphite particles was decreased to some extent. This is thought to
be because molten pitch preferentially adheres to edge planes of
the graphite particles which have a larger surface area than basal
planes.
* * * * *